The present invention relates generally to magnetic resonance (MR) imaging and spectroscopy. More particularly, the present invention relates to multi-channel phased array receiver coils for use in MR imaging or spectroscopy.
The magnetic resonance (MR) phenomena involves providing a fairly strong static magnetic field (polarizing field Bo, along the z-direction in a Cartesian coordinate system denoted as x, y, and z) throughout an image volume of the subject or area of interest (e.g., one or more anatomy of a patient being studied). Of the molecules comprising the subject or area of interest within the image volume, those nuclei having magnetic moments (i.e., those having an odd number of protons) attempt to align themselves with this static magnetic field. Such orientated nuclei, i.e., in a quiescent orientation, can be nutated (by controlled amounts) when a radio frequency, (RF) pulse (excitation field B1), which is in the x-y plane and which is tuned to the Larmor frequency, is applied in its vicinity. The presence of the RF pulse causes a net aligned moment, Mz, of the orientated nuclei to be rotated or xe2x80x9ctippedxe2x80x9d at a certain tipping angle into the x-y plane, to produce a net traverse magnetic moment, Mt. Once the RF pulse is terminated, the nutated or excited nuclei eventually return to their quiescent orientation and in the process emit certain MR echo signals, which can be detected and processed to form an MR image.
When utilizing these signals to produce MR images, linear magnetic field gradient pulses (gx, gy, and gz) along three mutually orthogonal axes are also applied in a predetermined sequence to spatially encode the echo signals, so as to produce a map or xe2x80x9cimagexe2x80x9d of the different nuclei populations (i.e., the various tissues) within a given image volume. Typically, the object to be imaged is scanned by a sequence of measurement cycles, in which the linear gradient pulses and the RF pulses are selectively superimposed on the static magnetic field in accordance with the particular localization method being used. The resulting set of received MR signals, also referred to as nuclear magnetic resonance (NMR) signals, are digitized and processed to reconstruct data representative of the volume of spatially encoded and nutated nuclei into an MR image, using one of many well-known reconstruction techniques.
Presently MR imaging or spectroscopy systems utilize radio frequency (RF) receiver coils to receive the echo signals emitted from the object of interest under study. The RF receiver coils may be of the type known as surface coils, which are typically smaller in size than remote coils and which are applied near, or on, the surface of a region of the object of interest (e.g., a specific anatomical portion of a patient, such as, a vertebrae, an elbow, etc.). Because the surface coil reception element can be located closer to the spins of interest, a given spin will produce a larger EMF in a surface coil than in a remote coil. The use of a surface coil also reduces noise contributions from electrical losses in the object of interest, relative to a remote coil, thereby resulting in MR images having a high signal-to-noise ratios (SNRs).
Surface coils are typically much smaller than remote coils in order to perform localized high resolution imaging of a small region of the object of interest, rather than an entire anatomical cross-section. A surface coil having a conductive loop or coil of diameter D provides the highest possible SNR for a volume of the object of interest directly below the surface coil and around approximately a depth D inside an infinite conducting half space. A surface coil of diameter D can only effectively image the region of the object of interest of lateral dimensions comparable to diameter D. Thus, surface coils, and more particularly, conductive loops, have a built-in field-of-view restriction.
As the conductive loops comprising the surface coils are decreased in size, the SNR of the MR images generated therefrom increases. However, as discussed above, when the loop size decreases (i.e., diameter D of the surface coil decreases), the field-of-view correspondingly decreases. Thus, using surface coils is a trade-off between resolution and field-of-view. Surface coils also tend to have non-uniform sensitivity to individual spins within its imaging volume, such that the received signals require additional compensation to address such inhomogeneity.
In order to take advantage of the high resolution (i.e., the high SNR) possible with surface coils while extending their field-of-view, a set of conductive loops or coils may be configured together in a given surface coil. With such a configuration, the high SNR of each single conductive loop or coil may be maintained, a larger overall field-of-view may be achieved, and simultaneous echo signal acquisition from each of the loops with uncorrelated noise therebetween may also be achieved. However, extreme care must be taken to minimize or eliminate interactions between the loops or coils (e.g., mutual inductances) through careful design of the geometry of and overlap between the loops or coils. Otherwise, mutual coupling between the loops will cause the SNR associated with each loop to degrade.
A surface coil having more than one conductive loop or coil and which is configured to minimize the mutual inductances therebetween are commonly referred to as a phased array coil or multi-channel phased array coil. Two- or three-channel phased array coils (i.e., a surface coil having two loops or three loops, respectively) may be constructed by careful adjustment of the overlap area between any two of the loops to minimize the undesirable mutual inductance(s). However, once four or more loops are included in a surface coil, designing the geometry or shape of such loops and/or the overlap areas between such loops to have simultaneously minimal mutual inductances has proven to be more difficult.
For example, when designing a four-channel phased array coil, although it is possible to minimize the mutual inductances between a given loop and each of its two adjacent loops (the loops to the right and left) by adjusting the overlapping areas therebetween, minimizing the mutual inductance between the given loop and the loop diagonal or opposite thereto is quite difficult. Thus, it is very difficult to design a four loop configuration that simultaneously minimizes all the mutual inductances between any pair of the loops.
Thus, there is a need for a four-channel phased array coil which provides a high SNR and an extended field-of-view by having all the mutual inductances between its loops be simultaneously minimized. There is a further need for a four-channel phased array coil configured to have uniform sensitivity to individual spins within the region of the object of interest being imaged. There is still a further need for a methodology for designing multi-channel phased array coils which satisfy the minimal mutual inductance requirement.
One exemplary embodiment relates to a surface coil for receiving magnetic resonance (MR) echo signals emitted from a region of an object of interest. The surface coil includes N coils configured in an array. Each of the coils has a geometric shape and overlaps with (Nxe2x88x921) coils to form an overlap area within the array. The geometric shape of each of the coils and the overlap area are configured to cause a mutual inductance between every pair of the coils to be less than 10 percent of the self-inductance of a single coil. N is equal to at least four.
Another exemplary embodiment relates to a method for simultaneously acquiring multiple channels of magnetic resonance (MR) signals for reconstructing into a single MR image. The method includes providing N antenna elements, each of the antenna elements including a geometric shape. The method further includes configuring the N antenna elements into a phased array. The phased array includes an overlap area formed by the overlap of the antenna elements with each other. The method still further includes approximately nulling a mutual inductance between every pair of the antenna elements. The overlap area is in the range of between 0 and 10 percent of a total area of the phased array and N is equal to at least four.
Still another exemplary embodiment relates to a four-channel phased array coil for receiving magnetic resonance (MR) echo signals emitted from an object of interest. The coil includes four antenna elements configured to respectively provide four channels of signal reception with negligible correlated noise with each other. Each of the antenna elements having a geometric shape. Each of the antenna elements overlaps with three other antenna elements to form an overlap area of between 0 and 10 percent of a total area of the four antenna elements. A mutual inductance between every pair of the antenna elements being less than 10 percent of the self-inductance of a single coil.